(1) Field of the Invention
The present invention relates to a process for producing solid oxide fuel cells each having an air electrode and a fuel electrode provided on opposite surfaces of a solid electrolyte plate, respectively.
(2) Related Art Statement
Recently, the fuel cells have attracted attention as a power generator. The fuel cell is a device capable of directly converting chemical energy possessed by a fuel to electric energy. Since this device is not restricted by the Carnot cycle, the device has essentially high energy conversion efficiency, and a variety of fuels such as naphtha, natural gas, methanol, coal-reformed gas and heavy oil can be used. Further, the device is less public nuisance, and the power-generating efficiency thereof is not influenced by the scale of the plant. Thus, the fuel cell is an extremely promising technique.
Particularly, since the solid oxide fuel cell (hereinafter referred to briefly as SOFC) works at high temperature around 1,000.degree. C., its electrode reaction is extremely active. Therefore, a noble metallic catalyst such as precious platinum is completely unnecessary. Further, the SOFC has small polarization and relatively high output voltage. Accordingly, the SOFC has far higher energy-converting efficiency as compared with other fuel cells. Furthermore, since structural materials are all solid, the SOFC is stable, and has long service life.
In order to produce a solid oxide fuel cell having an air electrode and a fuel electrode provided on opposite surfaces of a solid electrolyte plate, respectively, as one type of the above-mentioned solid oxide fuel cells, processes have been formerly known, in which a film of partially stabilized zirconia as a solid electrolyte is formed on an electrically conductive substrate as an air electrode by slurry coating, EVD, or plasma spraying. To the contrary, there are also known a process in which an air electrode and a fuel electrode are each formed by screen printing electrically conductive materials on an ion-conductive zirconia and firing them, and a process in which a film of green sheet made of ion-conductive zirconia and green sheets as electrodes are laminated one upon another, and then co-sintered.
However, among the above processes, the slurry-coating method and the plasma-spraying method have a problem in that a dense film of zirconia is difficult to obtain. On the other hand, the EVD method has problems in that the device is of a great scale, and a production cost becomes greater. Furthermore, the electrically conductive material-screen printing method has a problem in that since the zirconia film itself maintains strength, the strength of the fuel cell decreases. In addition, the co-sintering method has problems in that it is difficult to match the coefficient of shrinkage between the zirconia green sheet and those for the electrodes at the time of firing, and thus the shape stability after the firing becomes poor. Moreover, according to NGK's investigations, the co-sintering process has a problem in that a high resistance layer made of lanthane zirconate is formed at an interface between zirconia and the electrode to lower the power-generating efficiency.